The Recent as represented on the Texas coast may be separated chronologically into the Ingleside, Alazan, and Post-Alazan or Present times. Each subdivision is associated with a major climatic fluctuation, the Ingleside and Post-Alazan with the “first and second climate optimums”, respectively, and the Alazan with the “Little Ice Age”. Characteristic of the Ingleside and Present times, and possibly of Alazan time, are the associated coastal barrier and lagoon systems. The physiographic features of the Present coastal barrier system forming the outer Texas coast, manifest a direct relation to the climatic characteristics of the Present, favoring onshore movement of sediments. The sediments carried by mainland streams to the bay systems, then moved through the barrier passes by tidal currents, and contributed to a southwestward moving longshore current. Wave transport and eddy action transfer the sediments to an inshore drift current, which is reversible in direction with the longshore component of the local wind. The direction of the inshore drift current affects directly the orientation of foreshore features: spits, bays between cusps, and the associated backwash channels and ripple marks. Three basic profiles representing calm, normal, and storm wind conditions result from secretion or removal of beach sediments. Hurricanes, occurring at any one locality on the Texas coast about once every 25 years, produce great waves that flood the barrier system in the region where the storm track crosses the coastline. A hurricane flood flattens low foredunes, breaks through gaps in higher foredunes, forms washover fans and temporary passes, and carries the material across the island channels, depositing the sediments in front of back dune fields and in the lagoon. The dry portion of the present wind regime (that free of precipitation) transports the Gulf beach sands inland over the barrier system. Precipitation, occurring more often with northerly winds than with southerly winds, reduces the effect of the former on sand movement. SE is the prevailing wind direction and also the resultant effective direction causing Aeolian movement of sand over the barrier system, producing an over-all resultant NW movement. Blowout tongues, barchans, and seif dunes of the barrier system, also the dune fields and wind-cut ridges and furrows of the mainland, have developed trends in agreement with the NW direction of Aeolian sand movement. The NW trends are superimposed on a sand sheet that centers at 27°N latitude on the mainland and extends W from Laguna Madre. The sand sheet was probably the product of a different wind regime because of its non-agreement with the present direction of sand movement. A gradual change of physiographic features southwestward along the coast is the direct result of the different degrees of sand movement corresponding with a climatic range from humid in the vicinity of Sabine Pass to semi-arid on northern Padre Island. Desert-type dunes, the seifs and barchans typical of the great ergs of the world, are found in the semi-arid portion of the Texas barrier system. Of the entire continental shores of the United States, only along the Texas shores do such dunes develop for no other shore in the United States approaches the same degree of aridity. / text

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Cretaceous limestone, marl, and dolomite of Late Aptian to Middle Albian age crop out in the Wimberley area, a 5-minute by 10-minute quadrangle in central Texas situated in the dissected eastern margin of the Edwards Plateau. Formations exposed are the upper part of the Glen Rose, the Walnut, and the lower part of the Edwards. The Glen Rose, which crops out over 90% of the area, is subdivided into 7 informal members defined on mappability on aerial photographs. Six major step faults of the Balcones fault zone transect the area, displacing the strata downward to the southeast about 700 feet. The outstanding geomorphic features are the high relief hills and ridges south of the Blanco River, which are caused by dissection along the Edwards Plateau margin, and the deflections of Cypress Creek and Blanco River where they cross faults. / text

The Signal Hill Quadrangle is located astride the Balcones Escarpment southwest of Austin. Cretaceous rocks (Albian and Cenomanian) cropping out in the area include the upper two members of the Glen Rose Formation, the Bull Creek and Bee Cave Members of the Walnut Formation, the Kainer and Person Formations of the Edwards Group, and the Georgetown, Del Rio, and Buda Formations. Deposition represented by these formations ranges from supertidal to tidal-flat to open-shelf marine environments. At one location there is a basalt plug, probably of Senonian age. The youngest deposits in the quadrangle are those associated with Quaternary terraces and alluvial sands and gravels. The faults mapped are part of the Balcones Fault Zone, a system of en echelon, northeast-trending, predominantly normal, dip-slip faults. This system was probably active in the middle Tertiary. The Mt. Bonnell Fault is the most important fault in the quadrangle. It is one of the major faults of the Balcones Fault System, having been downthrown 170-350 feet to the southeast. The total displacement of all faulting in the map area is about 800 feet. / text

Youthful karst topography and entrenched drainage are two conspicuous geomorphic aids in interpreting the geologic history of the Waco Springs quadrangle. Three high angle, dip slip strike faults of the Balcones system, displaced toward the coast, have off-set all, and exposed most, of the southeastward dipping Comanchean and Gulfian rocks. Broad shallow anticlines are created by a "reversal of dip" on each downthrown fault block. Waco Springs, located in the southwestern part of the map area, has a separate groundwater reservoir from the remainder of the quadrangle, and its discharge is dependent on rainfall concentrated in central Comal County. / text

Bat Cave quadrangle lies on the maturely dissected eastern margin of the Edwards Plateau. Sinkholes, caves, and other karst features are common throughout the uplands. Approximately 1,050 feet of dominantly carbonate rock comprises the ten Cretaceous formations cropping out in the quadrangle. The upper part of the Glen Rose Formation, the Walnut, Kainer, and Person Formations, the Georgetown Limestone, Del Rio Clay, Buda Limestone, Eagle Ford Shale, Austin Limestone, and Taylor Clay are exposed. The oldest and youngest rocks crop out in the northwestern and southeastern corners, respectively. Five major, downthrown to the coast, high angle, normal faults of the Balcones fault system pass through the quadrangle. Together with 86 minor antithetic, synthetic, and cross faults, they produce 1,200 feet of stratigraphic displacement from northwest to southeast. Major faults trend between N. 40° E. and N. 60° E. Minor faults have more variable orientations, but tend to strike either east-west or N. 40° E. to N. 60° E. The Upper Member of the Glen Rose Formation supplies small amounts of fair to poor quality water to wells in the northern half of the quadrangle. The Kainer and Person Formations, which yield large amounts of good quality calcium bicarbonate water, are the principal aquifers in the southern half. / text

No description available.

The Eastern Shelf was a constructional platform developed on the margin of the sediment-starved Midland Basin during Late Pennsylvanian and Early Permian time. A mixed terrigenous-carbonate sedimentary province characterized the shelf during most of its history. Sediments were derived from highlands to the east and northeast. Along the outcrop in Eastland, Stephens, Young, and Jack counties, uppermost Pennsylvanian beds compose the Harpersville Formation, a boundary-defined rock stratigraphic unit within the Cisco Group. Harpersville facies extend westward into the subsurface 50 to 60 miles, where they grade into equivalent shelf margin carbonate and slope terrigenous facies. Preserved relief between the shelf margin and basin floor ranges from 600 to 1100 feet with dips of up to five degrees. Three depositional systems are recognized on the basis of gross lithologic composition and position relative to the shelf edges. They are the Cisco fluvial-deltaic system, the Sylvester shelf edge bank systern, and the Sweetwater slope system. The Cisco fluvial-deltaic system is composed of dip-fed fluvial-deltaic facies and associated strike-fed interdeltaic embayment facies. Eight deltaic lobe complexes have been mapped. The Sylvester slope system is composed of several slope wedges or fans each of which includes shelf margin, slope trough, and distal slope sandstone facies, as well as slope mudstone facies. Terrigenous sediments were transported across the shelf by prograding fluvial-deltaic channels, which locally extended through the shelf edge bank system and onto the slope where submarine fans were constructed into the basin. The Eastern Shelf prograded into the Midland Basin by local upbuilding through fluvial, deltaic, and shelf edge bank deposition contemporaneous with outbuilding by slope fan deposition. Sites of shelf construction shifted through time in response to sedimentary and structurally controlled abandonment of delta lobes. Extrabasinal controls such as eustatic sea level changes were of secondary importance in developing the depositional fabric of the shelf. / text

No description available.

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